This invention relates to the field of film-forming novolak resins, to photosensitive compositions comprising said novolak resins and to a process for forming an image on a substrate utilizing said photosensitive compositions.
Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The photoresist coated on the substrate is next subjected to an image-wise exposure to radiation.
The radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation exposed or the unexposed areas of the photoresist.
The trend towards the miniaturization of semiconductor devices has led to the use of new photoresists that are sensitive to lower and lower wavelengths of radiation and has also led to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization.
There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating, thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited.
On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution (e.g. a deprotection reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying surface is uncovered.
Positive working photoresist compositions are currently favored over negative working resists because the former generally have better resolution capabilities and pattern transfer characteristics. Photoresist resolution is defined as the smallest feature, which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of less than one micron are necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate. This becomes even more critical as the push toward miniaturization reduces the critical dimensions on the devices.
The principal ingredients in many of the conventional (e.g., i-line) positive-working photoresist compositions are an alkali-soluble novolac resin as the film-forming constituent and a quinone diazide compound as the photodecomposable or photosensitive constituent in the form of a mixture or a condensation product of both. Since the sensitivity, resolving power, etc. in the patterning work heavily depend on the kinds and proportion of these two constituents and the manner of combining them in the photoresist composition as well as on the procedure of development after exposure to light, various attempts and proposals have been made hitherto in respect of manufacturing of the photoresist compositions and the way of using the photoresist composition in the photolithography. For example, Japanese Patent Kokai No. 58-17112 teaches that the sensitivity of a positive-working photoresist composition comprising a cresol novolac resin as the film-forming constituent can be improved by suitably selecting the proportion of the cresol isomers in the cresol used in the preparation of the cresol novolac resin.
Typically, in photosensitive compositions, a resin having good thermal stability usually gives a pattern having poor resolution. Likewise, a resin having good resolution has poor thermal stability. A resin system that has both the properties of high thermal stability and high resolution are desirable. The present invention provides such a resin system. The present invention has resulted in finding that the combination of a resin for high thermal stability with a resin for high resolution together produce a resin which when used in photoresists results in preserving both the key properties (thermal stability and high resolution).
U.S. Pat. No. 4,731,319, issued Mar. 15, 1988, to Kohara et al., discloses positive-working photoresist composition comprising: (A) 100 parts by weight of a cresol novolac resin as a film-forming constituent; and (B) from 25 to 60 parts by weight of a naphthoquinone diazide sulfonic acid ester as a photosensitive constituent, the cresol novolac resin being a combination composed of: (A-1) a first cresol novolac resin having a weight-average molecular weight of at least 5000 and produced from an isomeric mixture composed of 60 to 80% of m-cresol and 40 to 20% of p-cresol; and (A-2) a second cresol novolac resin having a weight-average molecular weight not exceeding 5000 and produced from an isomeric mixture composed of 10 to 40% of m-cresol and 90 to 60% of p-cresol, in such a proportion that the overall cresol moiety in the component (A) is composed of from 30 to 46.5% of the m-cresol moiety and from 70 to 53.5% of the p-cresol moiety.